When mechanical rotation power is transformed at a determined angular velocity ω1 to another at a different angular velocity ω2, the quotient between both is called gear ratio. The speed variation mechanisms, or continuously variable transmission, aim that this gear ratio varies continuously and progressively.
Continuously variable transmissions can be classified in three large groups: A) Mechanisms that transmit the rotation directly from the input shaft to the output shaft via an intermediate element which, by friction, is driven by the input shaft and, in turn, drives the output shaft; this group includes v-belt speed variators, toroidal variators, conical roller variators, etc. B) Systems which transform the mechanical rotation power from the input shaft into another form of energy that is easier to handle and then again transform it into mechanical rotation power in the output shaft; this group includes hydrostatic variators, torque converters, pairs of electric machines, one working as a generator and the other as an engine, systems that transform the rotary movement into another oscillatory movement and then again generate a rotary movement with this. C) Systems which combine mechanisms from the previous groups with one or more differential mechanisms.
Group A mechanisms allow the continuous progressive variation of the gear ratio from a minimum value to a maximum value, both of the same sign, i.e. they do not allow the rotation direction to be reversed, due to the fact that to pass through zero gear ratio the friction element should drive one of the shafts on a null radius with infinite tension; indeed there is a minimum close to which the friction element slides and the mechanism stops working. The energy output is poor compared with those of gear transmissions due to a friction element.
Those of group B, if correctly designed, can reverse the rotation direction but also have a poor mechanical output compared to those of gear transmissions, due to the fact that losses accumulate in each transformation of power to another form.
Group C) mechanisms, i.e. the combination of one or more differential mechanisms with a speed variator, have been applied with two possible purposes: to obtain a mechanism that allows the variator's rotation direction to be reversed starting from a variator that initially did not allow this. To divide the power in two parts, one is transmitted via the variator and the other is transmitted via gears, which achieves a smaller variator and a greater mechanical output of the mechanism as a whole.
These mechanisms can, in turn, be classified in two subgroups: those that simply combine a variator with one or more differentials and those which add an external gear change to increase the variation range of the basic mechanism.
Within the mechanisms limited to combining a variator with a differential, there are several patents that disclose how to reduce the power transmitted by the variator, for example U.S. Pat. No. 1,762,199; these mechanisms are known as “power split”. Others such as, for example, U.S. Pat. No. 1,833,475, FR091705, U.S. Pat. No. 2,745,297, ES2142223 disclose mechanisms that allow the rotation direction to be reversed, these mechanisms are known as “power recirculating”.
Differences between some patents and others stem from the type of variator, from the type of differential and from the transmissions adopted to connect them.
It is not possible to obtain both effects simultaneously. Mechanisms that reverse the rotation direction (“power recirculating”) amplify the power which circulates through the variator; mechanisms that reduce the power that circulates through the variator (“power split”) do not allow the rotation direction to be reversed.
Mechanisms such as those disclosed in the previous patents, achieve the mechanism's overall gear ratio in function of the variator gear ratio according to an equation of type:
      τ    =                  a        +                              b            ·            r                    ⁢                                          ⁢                      r            min                              ≤      r      ≤              r        max              or      τ    =                            1                      a            +                          b              ·              r                                      ⁢                  r          min                    ≤      r      ≤              r        max            Where:
τ is the mechanism's overall gear ratio, i.e. the angular velocity of the output shaft divided by the angular velocity of the input shaft.
r is the speed variator's gear ratio: this ratio varies 10 between a minimum value rmin and a maximum value rmax, both of the same sign in the event that the variator does not have the characteristic of being able to reverse the rotation direction.
a and b are constants which depend on the fixed gear ratios, on the differential's characteristics and the way in which different components are interconnected.
In these conditions, the power transmitted by the variator is a function of the mechanism's gear ratio at all times, if constants a and b are chosen so that the point at which this power is at its maximum, this value is the minimum value possible so that:
For mechanisms with a group A variator:
      λ    max    =                    β        -        1            β        ·          rang              rang        -        1            And for mechanisms with a group B variator:
      γ    max    =            β      -      1              β      +      1      Where:
γmax is the power fraction transmitted by the mechanism that passes through the variator, i.e. the power transmitted by the variator divided by the total power transmitted by the mechanism when the gear ratio makes this value the maximum.
β is the variability in the mechanism gear ratio, i.e. the maximum gear ratio that can be obtained divided by the minimum.
rang is the range of variability of the variator, i.e. rmax/rmin.
Mechanisms have also been proposed that combine two or more differentials with a variator and a greater or lesser quantity of countershaft ratios between these elements; examples of this type of mechanism are disclosed in patents U.S. Pat. No. 2,384,776, U.S. Pat. No. 4,936,165, ES2190739, FIGS. 17, 18 and 19 illustrate the mechanisms respectively proposed in each of said patents in block diagrams. They all manage to obtain the mechanism's overall gear ratio in accordance with the variator gear ratio of type:
            ω      o              ω      i        =      τ    =                            a          ·          r                +        b                              c          ·          r                +        d            
Where τ is the mechanism's global gear ratio, r is the variator gear ratio and a, b, c and d adopt values according to the characteristics of the differentials and countershaft ratios.
Despite permitting an advantage with respect to those that use a single differential, these mechanisms have the drawback of the losses arising in the countershafts.
There is a known solution that eliminates countershafts and which has been disclosed in patent U.S. Pat. No. 6,595,884 (the block diagram of the mechanism claimed therein is shown in FIG. 20). Despite eliminating countershafts, this mechanism still has the drawback that only part of the power passes directly from the input shaft to the output shaft crossing a single differential, the rest has to cross two differentials which mean an accumulation of losses.
Mechanisms that add an external gear change aim to achieve wider overall variability of the mechanism and, likewise, they have to reduce the variability of the basic mechanism to the minimum, which means, if the components are suitably chosen, the power fraction that circulates through the variator can be reduced as much as desired, to add a sufficiently large number of gears in the external change.
Two forms of increasing the range by a 30 external gear change have been disclosed, the first as disclosed in patent U.S. Pat. No. 5,167,591, whose operation can be summarised as follows: supposing that we start with the minimum gear ratio, it is first changed progressively until reaching the maximum, at this point the transmission is disconnected by clutch, during this disconnection it is changed to the minimum gear ratio and the following gear is connected repeating the cycle.
The drawback of this system is that during the gear change the system remains disconnected for a time (out of gear); this time is not negligible as the variator should pass from its maximum gear ratio to the minimum or vice versa before being able to be reconnected (put in gear); on the other hand this solution allows the system to be designed with as many steps as desired and allows any progression between steps.
Another solution is disclosed in patent U.S. Pat. No. 5,643,121; the idea consists of alternatively connecting one of two shafts of the differential to the output shaft (the third shaft is the mechanism input), the alternation obtaining a gear ratio change. The gear ratios are determined so that during the transition both ratios can be kept in gear simultaneously using different clutches (synchronous change) meaning that the transition time can be negligible and, furthermore, the power transfer to the output shaft is not lost during the transition. The system would equally function if the input shaft was the output shaft and vice versa. Although more than two steps can be connected (as the author comments in the patent) they are all interdependent and the system can only be optimized for the first two.
Thus in a system with two steps such as the example shown in the patents, a variability of 6.25=2.52 is obtained with the same maximum fraction of power through the variator than if the variability was 2.5 (71.4%). But increasing the number of steps until the infinite would only achieve a 50% reduction in the fraction of power through the variator.
Patent U.S. Pat. No. 5,643,121 also discloses the form of reversing the rotation direction, for which it adds a second differential; this second differential idles in all regimes except those which allow the change in rotation direction, wherein the second differential subtracts the speeds of the two arms of the first differential to allow variation from a positive value to a negative value passing through zero. During the regime that allows the rotation direction to be reversed, the system responds to a scheme such as that of FIG. 21. The input power passes to the output crossing the two differentials and with recirculation through the variator.
All these systems require for their embodiment a control system that, starting from the gear ratio one wants to obtain at any time, acts on the variator element. This action is performed using mechanisms actuated by electric motors or pneumatic or hydraulic cylinders, etc.